Sync in ical

Calendars are not actually sync-ed at all.

If a calendar in not being updated, it is likely because it is not a Master calendar but a strictly private one that exists only on your computer.

Instead, the Master Calendar is placed on a server visible on the internet and all appropriate users subscribe to it. If you have more than one Apple device, you can easily use a master calendar on your iCloud account for this purpose, even for readers who are not Apple-connected.

Keep your Calendar up to date with iCloud - Apple Support

Mac Basics: Calendar keeps your appointments - Apple Support

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Check

Calendar > Preferences > Accounts ...

... to be sure the information there is still valid, and that the calendar is [√] Enabled

If you last updated to High Sierra, know that Apple File Protocol (AFP) has been removed, and some network connections may need to be set up again. be sure you are using Secure Socket Layer (SSL).

Have you used:

View > Refresh Calendars?

Have you Restarted your Mac?